1. Field of the Invention
This invention relates generally to GMR read heads of the bottom spin valve structure and, more particularly, to a spin-filter type of bottom spin valve having an high-conductance layer and an ultra-thin CoFe free layer which has positive magnetostriction and high output.
2. Description of the Related Art
Magnetic read heads whose sensors make use of the giant magnetoresistive effect (GMR) in the bottom spin-valve configuration (BSV) are being increasingly required to read information recorded on magnetic media at ultra-high area densities (e.g. >45 Gb/in2). The typical BSV sensor configuration includes (in vertically ascending order) a pinning layer, a pinned layer, a conductive spacer layer, a ferromagnetic free layer and a capping layer. Sensing current is introduced into and extracted from this configuration by laterally disposed leads. Again, typically, the pinning layer is a layer of antiferromagnetic material (AFM) which pins (fixes in space) the magnetic moment of the pinned layer (typically a layer of ferromagnetic material) in a direction normal to the plane of the air-bearing surface (ABS) of the sensor. The magnetic moment of the ferromagnetic free layer, not being pinned, is free to rotate with respect to that of the pinned layer under the influence of external magnetic fields and it is those rotations that cause the resistance of the sensor, R, to vary (dR) and, in combination with the sensing current, to produce an electrical signal. The GMR effect, which is relied upon to give maximum resistance variations, dR, for given rotations of the free layer magnetic moment, is a result of the scattering of conduction electrons in the spacer layer by the surfaces of the pinned and free layers that bound it. This scattering is spin-dependent and a function of the relative orientations of the two magnetic moments.
In order for the dR to be reproducible and invariant under symmetric changes in the external field, the magnetic moment of the free layer should return to the same position (the bias point) when no external magnetic signals are present (the quiescent state). The bias point of the free layer is typically made to be perpendicular to the pinned moment of the pinned layer, ie. in the plane of the ABS.
To be capable of reading ultra-high area densities, the BSV sensor must be able to resolve extremely high linear bit densities, bits-per-inch, (BPI) and track densities, tracks-per-inch, (TPI), which, in turn, requires that it have an extremely narrow trackwidth and ultra-thin free layer (thickness <20 angstroms) to maintain high signal output. Unfortunately, as the free layer is made increasingly thin, it becomes difficult to obtain a controllable bias point, a high GMR ratio (dR/R) and good softness (low coercivity). Utilizing synthetic antiferromagnetic (SyAF) pinned layers (ferromagnetic layers coupled with their magnetic moments antiparallel) can reduce magnetostatic fields between the pinned and free layers which adversely affect the biasing; but if the free layer is sufficiently thin, even the magnetic fields produced by the sensing current have an adverse affect.
The prior art teaches several methods for increasing the GMR ratio of a BSV sensor. Pinarbasi (U.S. Pat. No. 6,201,671) teaches the formation of a nickel oxide (NiO) pinning layer formed on a tantalum oxide (TaO) seed layer, which offers an improved GMR ratio for both SyAP pinned layers and simple ferromagnetic pinned layers. The free layer is a 70 angstrom thick layer of NiFe. Pinarbasi (U.S. Pat. No. 6,208,492) teaches the formation of an iridium manganese (IrMn) pinning layer formed on a bilayer seed layer which is a layer of nonmagnetic metal formed on a layer of metallic oxide. The free layer is a CoFe/NiFe bilayer in which the CoFe is 15 angstroms in thickness and the NiFe is 45 angstroms in thickness. Pinarbasi (U.S. Pat. No. 6,404,606) teaches the formation of an improved seed layer structure for a PtMn pinning layer in which the seed layer includes a first layer of aluminum oxide, a second layer of nickel manganese oxide and a third layer of tantalum. The seed layer increases the pinning and exchange coupling fields between the pinning and pinned layers which improves recovery of the pinned layer magnetic moment if it subjected to temperatures above the blocking temperature and reversal of its magnetic moment. Gill (U.S. Pat. No. 6,400,536) teaches the formation of a free layer with an improved uniaxial anisotropy. The layer is a triple layer wherein each of the three layers has a different uniaxial anisotropy constant and the three layers are mutually exchange coupled. An exemplar of the free layer consists of a 10 angstrom CoFe layer on which on which is formed a 30 angstrom NiFe layer and on which is formed a 10 angstrom CoFe layer. Huai (U.S. Pat. No. 6,222,707) teaches a bottom or a dual spin valve with a seed layer on which is grown an antiferromagnetic (AFM) pinning layer or a synthetic antiferromagnetic (SyAF) pinned layer. When used to grow the AFM layer, the seed layer improves texture of the fcc lattice structure. When used to grow the SyAF layer, the seed layer improves exchange coupling. Fukuzawa et al. (U.S. Pat. No. 6,338,899) discuss the advantageous nature of oxidized metallic layers and also teach the formation of TaO layers in a variety of spin valve configurations.
The prior art cited above have approached the matter of improving BSV performance differently, either by improving the performance of the pinned/pinning layer by the use of novel seed layers or by improving the performance of the free layer with novel composite structures. None of the art cited has specifically addressed the problem of free layer biasing for an ultra-thin free layer (less than 20 angstroms in thickness). To overcome this significant problem, a spin-filter spin valve (SFSV) configuration has been introduced (see H. Iwashaki et al., “Spin Filter Spin Valve Heads With Ultarthin CoFe Free Layers,” Abstract BA-04, 1999 Intermag. Conference) in which the free layer is placed between the usual Cu spacer layer and an additional high-conductance-layer (HCL). This configuration reduces the sense current field in the free layer by shifting the sense current center towards the free layer. This results in the sense current producing a small bias point shift. In addition, the SFSV configuration allows the use of an ultra-thin CoFe free layer which, when combined with a properly formed HCL, has an advantageous small positive magnetostriction combined with a high output.